Historically, dimple patterns for golf balls have had a variety of geometric shapes, patterns, and configurations. Primarily, patterns are laid out in order to provide desired performance characteristics based on the particular ball construction, material attributes, and player characteristics influencing the ball's initial launch angle and spin conditions. Therefore, dimple pattern development is a secondary design step that is used to achieve the appropriate aerodynamic behavior, thereby tailoring ball flight characteristics and performance attributes.
Aerodynamic forces generated by a ball in flight are a result of its velocity and spin. These forces can be represented by a lift force and a drag force. Lift force is perpendicular to the direction of flight and is a result of air velocity differences above and below the rotating ball. This phenomenon is attributed to Magnus, who described it in 1853 after studying the aerodynamic forces on spinning spheres and cylinders, and is described by Bernoulli's Equation, a simplification of the first law of thermodynamics. Bernoulli's equation relates pressure and velocity where pressure is inversely proportional to the square of velocity. The velocity differential, due to faster moving air on top and slower moving air on the bottom created by the ball's spin, results in lower air pressure on top and an upward directed force on the ball.
Drag is opposite to the direction of flight and orthogonal to lift. The overall drag force on a ball is attributed pressure drag and viscous or skin friction drag. A sphere is a bluff body, which is a somewhat inefficient aerodynamic shape. As a result, the accelerating flow field around the ball causes a large pressure differential with high-pressure forward and low-pressure behind the ball. The low pressure area behind the ball is also known as the wake. In order to minimize pressure drag, dimples provide a means to energize the flow field and delay the separation of flow, or reduce the wake region behind the ball. Skin friction is a viscous effect residing close to the surface of the ball within the boundary layer.
The industry has seen many efforts to maximize the aerodynamic efficiency of golf balls, through dimple distribution and other methods, though they are closely controlled by golf's national governing body, the United States Golf Association (U.S.G.A.). One U.S.G.A. requirement is that golf balls have aerodynamic symmetry. Aerodynamic symmetry allows the ball to fly with a very small amount of variation no matter how the golf ball is oriented when tested. Preferably, dimples cover the maximum surface area of the golf ball without detrimentally affecting the aerodynamic symmetry of the golf ball and the ability of the ball to roll smoothly.
In attempts to improve aerodynamic symmetry, many dimple patterns have been developed based on geometric shapes. These may include circles, hexagons, triangles, and the like. Other dimple patterns are based in general on the five Platonic Solids including icosahedron, dodecahedron, octahedron, cube, or tetrahedron. Yet other dimple patterns are based on the thirteen Archimedian Solids, such as the small icosidodecahedron, rhomicosidodecahedron, small rhombicuboctahedron, snub cube, snub dodecahedron, or truncated icosahedron. Furthermore, other dimple patterns are based on hexagonal dipyramids. Dimple properties such as number, shape, size, volume, edge angles and arrangement are often manipulated in an attempt to generate a golf ball that has improved aerodynamic properties.
Furthermore, secondary surface texture has been suggested to augment the dimples and further refine the aerodynamic properties of the ball. In fact, early golfers found that the feathery golf balls flew better after being played for a while. They then began to purposely roughen the surface to created improved aerodynamic properties.
Similarly, U.S. Pat. No. 4,787,638 to Kobayashi discloses a golf ball with a plurality of first dimples arranged substantially uniformly on the outer surface of the ball. The ball also includes a plurality of indentations which are smaller than the dimples and are also arranged substantially uniformly on the outer surface and inside the surfaces of the dimples. The indentations may be formed by grit blasting. Likewise, U.S. Publication No. 2012-0301617 teaches essentially the same micro surface roughness over the surface of the golf ball to affect aerodynamic properties of the ball.
U.S. Pat. No. 8,329,081 to Morgan discloses a method of forming a golf ball with secondary surface texture created on the fret areas of a ball. The secondary surface texture is created on the golf ball hob prior to the primary dimples being formed into the hob. When the dimples are formed, they largely obliterate the secondary surface texture except for the fret area and the perimeter of the dimples.
U.S. Pat. No. 6,569,038, to Sullivan discloses a ball having dimples with structures therein to energize or agitate the airflow over the dimpled surface to increase the aerodynamic performance of the ball. These structures include sub-dimples and radiating convex or concave arms emanating from the center of the dimple.